Customarily, an apparatus that is used for plasma cutting is constructed as shown in FIG. 1. More specifically, a plasma torch in such an apparatus is provided with a centrally positioned electrode 1 and a confining nozzle 4 that is arranged to surround the electrode with a spacing therefrom which defines a passage 3 for a plasma gas 2. And, there is provided a plasma power supply 9 which is connected across to the electrode 1 and a workpiece 5, and further to the confining nozzle 4 via a switch 16. A cutting operation using such an apparatus is carried out by passing a plasma gas 2 through the passage 3 while generating a plasma arc between the electrode 1 and the workpiece 5 to produce a high temperature plasma gas stream 6 that is narrowly pinched by means of the confining nozzle 4 so as to project onto a given portion of the workpiece material 5. That portion of the workpiece 5 is thereby molten and removed.
By the way, it is typical in the plasma cutting that a quality of cutting may be enhanced by selecting a suitable plasma gas 2 with respect to a workpiece material 5. For example, by using an oxygen gas in cutting a mild steel, it has been known that an operation can be performed to yield a smooth surface of cutting with a dross less attached thereto and yet at an elevated cutting rate.
Also, in cutting a stainless steel material, it has been known that an inert or a reducing gas such as a nitrogen, argon+hydrogen or nitrogen+argon+hydrogen gas can be used to give rise to a smooth cutting surface and, when the workpiece thickness is not less than 6 mm, with a dross less attached thereto and with a high quality of cutting.
However, in case the workpiece is a thin stainless sheet material that is of a thickness less than 6 mm, it has been recognized that such a high quality of cutting is no longer obtainable.
More specifically, if it is attempted to cut a stainless sheet material that is so thin with an inert or a reducing gas as the plasma gas, a large quantity of the dross will be attached to the cut workpiece surface and cannot at the least be peeled. As a result, a subsequent operation such as to grinding off the dross securely deposited onto the cut workpiece surface with a grinder has hitherto been required. Further, since a large quantity of the dross is adhered to the cut workpiece material, it has been found that a heat is conducted from the adhered dross into the workpiece material, thus giving rise to a large thermal deformation thereof. For those reasons, the use of an inert or a reducing gas as the plasma gas for cutting a stainless steel workpiece material has been found to be virtually impractical.
Also, although it is known that if a stainless sheet material is cut using an oxygen gas as the plasma gas, a considerable improvement can be achieved with respect to the problems of a dross that is tending to deposit onto a rear side of the workpiece material, it is also known that the cut surface of the workpiece is severely oxidized by the oxygen gas and consequently becomes blackened and rough. Also, if the cutting rate is set fast, a large amount of the dross will be blown up above the upper surface of a cutting portion of the workpiece material and will build up thereon, thereby reducing the quality of a resultant cut. Conversely, if the cutting rate is set slow, the input heat into the workpiece material will be enlarged, thus enlarging the thermal deformation thereof. Then, in addition, with the oxidation of the cutting surface being promoted, the quality of a resultant cut will still further be lowered. Thus, it has commonly be recognized that it is difficult to properly set the cutting rate.
As set forth above, if a stainless steel sheet material of a thickness not greater than 6 mm is to be cut by plasma cutting, it may be clearly apparent that an effort to variously alter the plasma gas has not result in a good quality of the plasma cut.
Also, it should be noted that another type of the plasma cutting apparatus has been proposed, as shown in FIG. 2, in which the plasma torch is provided with an assisting nozzle 7 that is mounted around the confining nozzle 4 with a spacing therefrom defining a blow-out passage which is concentrically arranged with the confining nozzle 4. With the assisting nozzle 7 so arranged, the circumference of the plasma gas flow is shielded with a secondary gas 8 which is designed to enable the gaseous atmosphere around a cutting area to be controlled.
An example of the preceding type of the plasma cutting apparatus has been proposed as has been disclosed, for example, in Japanese Unexamined Patent Publication No. Sho 59-229,282, if the plasma gas 2 is composed of an oxygen gas, the secondary gas 8 is composed of an oxygen gas as well to shield the cutting area from the atmosphere. Such an example has been contemplated to cut a mild steel workpiece with a high quality of cutting by holding the oxygen purity in the cutting environment to be high.
There has also been proposed a modified system in which both the plasma gas 2 and the secondary gas 8 are composed of an inert gas so that the cutting area may be prevented from being oxidized by the atmosphere.
There has further been proposed another modified system in which the plasma gas 2 is composed of an inert gas and the secondary gas is constituted by a gas that is rich in the oxygen concentration. By feeding the oxygen gas into the cutting area, this modification has been intended to improve the quality of cut of a mild steel workpiece with the oxygen gas while elongating the utility life of the electrode with the inert gas.
On the other hand, the purpose of using the secondary gas has also be made, as is apparent from Japanese Unexamined Patent Publication No. Hei 05-84,579, to aim in protecting the confining nozzle from the dross that may be blown up from the workpiece material at the times of piercing and cutting thereof. In this case, the purpose of using the secondary gas is not to alter the gaseous atmosphere in the cutting area, but to deal with a dross scattering from the workpiece material by thrusting back the dross to prevent it from being adhered onto the confining nozzle.
By the way, the usage of the secondary gas in these plasma cutting methods has hitherto been set to an amount that is as little as possible. This has come from the economical requirement that the amount of its consumption should be minimized as well as from the emphasized requirement that the secondary gas which is confined and blown out should not interfere with the plasma gas and then should not either disturb or cool the plasma gas. Also, a care has been taken to ensure that the secondary gas flow should be oriented externally as much as possible so as not to interfere with the plasma gas flow.
At this point, it should be noted that the reasons why the secondary gas flow and the plasma gas flow have been controlled not to interfere with each other are that the conventional plasma cutting technique is designed to cut a workpiece by melting and removing a predetermined portion thereof with a high temperature and high speed plasma flow, and that the plasma gas flow flushed from the confining nozzle, if disturbed and cooled there, would lower the cutting capacity.
In this manner, the conventional plasma cutting method using a secondary gas is characterized by only a small usage thereof and has thus utilized, basically just as in a form in which no secondary gas is used, a high temperature and high speed flow of the plasma. For this reason, it should be apparent that it has not been expected in the plasma cutting technique to largely improve the quality of cutting a stainless sheet workpiece material of a thickness that is not greater than 6 mm.
On the other hand, if the rate of flow of the secondary gas is increased, a pilot arc as required in starting a plasma cutting operation will be disturbed by the secondary gas; hence the ignitionability in plasma cutting will be deteriorated. Also, due to the pinch effect for the plasma flow by means of the secondary gas flow, the arc voltage may be elevated and, especially immediately after piercing, an excessive arc voltage may be created, thus making it highly susceptible for an arc to be extinguished.
In this manner, since an increase in the rate of the secondary gas flow largely influences the ignitionability at the starting time of a plasma cutting operation, a mere increase in such a rate of flow alone has hitherto made it difficult to operate a plasma arc cutting apparatus satisfactorily.
Accordingly, it is an object of the present invention to provide a plasma cutting method which is capable of improving the quality of a resultant cut when cutting a stainless steel sheet material of a thickness that is not greater than 6 mm while improving the ignitionability at the starting time of a plasma cutting operation.